Two types of earthworm, one black and one red-brown, inhabit the woods

okay here is my reasoning why i chose C

Since, in 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods.

The population of black earthworms is now almost equal to that of the red-brown earthworm, i.e. the population of black worms increased and the population of brown worms stagnated

Local ecologists say it is the blackening of the woods.

Ecologists`s claim implies that emissions enabled black worms to use their color as a comouflage.

To strengthen this claim one need to find a factor that could have negative affect on black worms. so based on option (C) despite favourble conditions for the red brown worm, its population didn`t grow whereas for black worm though climate conditions were less favourable, their black camouflage gave them opprtunity for survival and growth in black woods. hence C

Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.

A and B are good answers, but C is better.

A strengthens the argument because it says the red worm population is falling. One possible explanation is the black worms are now better camouflaged so the birds are eating more red worms.

But A isn’t ideal because there are other plausible explanations. Maybe there is a disease that’s killing red worms. We don’t know. A is tempting us to assume that the blackening of the forest is why the red worm population is falling but we don’t really know that for sure. So maybe this fact strengthens, or maybe it’s just a coincidence.

B is good, too, but it isn’t perfect. It says birds prefer black worms so you you would expect them to eat more black worms. Yet, the black worm population is rising. So we’re tempted to assume the birds would rather eat black worms but they aren’t doing so because the black worms aren’t visible so the birds settle for eating red. Okay, maybe. Or maybe the birds are still eating tons of black worms. We know the population of red and black is now equal but we don’t know that there are more black worms. Maybe they’re equal entirely because the population of red worms fell. Is that because the birds can see the red worms and are eating more of them? That’s a fair supposition but maybe something else, like disease or climate change, is causing the red worm population to fall.

And that takes us to C. C is best because it is ruling out these alternative explanations. A and B are plausible explanations, but they leave other possibilities open. C forecloses those possibilities.

This one is really hard, I would not fault anyone for picking A or B. We see a lot of questions where all the answers are bad and we have to find the least bad. Here, we have to pick the best of three pretty good answers.

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I chose B as my answer because of the following reasoning.

To strengthen an argument that A causes B, we should remove any possibility of C causing B.
A = Forest turning back
B = Black earthworm population increasing
C = The birds DO NOT prefer black earthworms over red earthworms

So, when we remove another possible factor (C) that could cause (B), it strengthens the argument .
The population of black earthworms have increased significantly NOT because the birds DO NOT prefer black earthworms.

OR

The population of black earthworms have increased significantly even though the birds prefer to eat black earthworms.

If the birds do not prefer black earthworms, then, now that the forest has turned black, red earthworms have lost their camouflage, so birds are eating more red earthworms. The population of red earthworms has decreased and so the number or black earthworms is almost equal to the number of red earthworms now.
Basically, the equality of the number of black and red earthworms is not because birds are eating red earthworms more, but because black earthworms are getting camouflage now that the forest has turned black.
So, I feel statement B also strengthens the argument.

Can anyone please explain why this reasoning is wrong? Is it too far fetched?

(B) could strengthen that "blackening of the woods is responsible" but it does not strengthen "blackening of the woods is SOLELY responsible".

Check out my detailed post on this: https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html/2015/1 ... onclusion/
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This is a good point. One more thing to consider is why isn't the blackworm population greater than the red worm population, given the blackening of the woods. There must be a reason the red worm population is not tapering off. Option C is the only option that gives a possible reason for the red worm population to equilibrate w/o going down. If option A is true then the black worm population would eventually be much larger than the red worm population.

Therefore C.

Hey experts

KarishmaB GMATNinja mikemcgarry ExpertsGlobal5 AjiteshArun

Could you explain why option D is incorrect? Isn't option D indirectly saying that lifespan did not cause it. Hence we are eliminating one possible cause. Hence, it is strengthening the argument.

Initially, number of Red worms was 5 times that of Black worms (because of better camouflage for Red). Now the two are almost equal, only because camouflage favours Black.
What will help strengthen it? By saying that nothing else changed between the two (e.g. red did not get infected by a virus that attacks red worms only).

A statement that gives us data about both together as a group will not make any distinction between the two. (D) says that avg earthworm lifespan has not changed. So it doesn't give any distinction between the numbers of the two. What affects (or doesn't affect) one affects (or doesn't affect) the other in the same way so it is not useful.
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